This invention relates generally to making downhole measurements during the drilling of a borehole to recover natural deposits of oil or gas and, more particularly, to using continuous downhole measurements to directionally drill the borehole.
Oil and gas are commonly recovered from natural mineral deposits in subsurface geologic formations in the earth""s crust. Drilling rigs at the surface are used to bore long, slender boreholes into the earth""s crust to the location of the subsurface oil or gas deposits to establish fluid communication with the surface through the drilled borehole. The downhole drilling equipment used to drill boreholes may be directionally steered to known or suspected oil or gas deposits using directional drilling techniques, and the direction and orientation of downhole survey instruments are monitored at discrete survey stations along the borehole.
Surveying of boreholes is commonly performed using downhole survey instruments. These instruments typically contain sets of three orthogonal accelerometers and magnetometers which are coupled within a bottom hole assembly (BHA), which is in turn coupled in the drillstring from 20 to 200 feet above the drill bit. These survey instruments are used to measure the direction and magnitude of the local gravitational and magnetic field vectors in order to determine the azimuth and the inclination of the borehole at each survey station within the borehole. Generally, discrete borehole surveys are performed at survey stations along the borehole when drilling is stopped or interrupted to add additional joints or stands of drillpipe to the drillstring at the surface.
The trajectory of drilled boreholes within any segment of interest is generally determined by the mode of drilling and by the configuration of the drilling equipment that is used to drill the borehole at that segment of interest. In directional drilling using a bent sub and a mud motor, there are two modes of drilling that produce distinctive borehole trajectories. The first mode involves rotation of the entire drillstring, including the BHA that contains the survey instruments, and the drill bit that is coupled to the bottom (leading end) of the drillstring. This mode of drilling, with the bent sub in its aligned or straight configuration and the mud motor inactive, is known as xe2x80x9crotating.xe2x80x9d The second mode of drilling has the bent sub in its deployed or angular configuration with the drill bit being rotated by the active mud motor instead of rotating the drillstring. The mud motor is powered by pressurized drilling mud pumped down the hollow interior of the non-rotating drillstring. This mode of drilling is known as xe2x80x9csliding.xe2x80x9d Rotating produces a generally linear trajectory of the drilled borehole, although there are typically borehole deviations from true linear trajectory due to the effects of gravity, geologic heterogeneities, misalignment between the BHA and the borehole, stiffness of the drillstring and transition effects that occur due to switching from one drilling mode to the other. Drilling while sliding produces a curved trajectory of the drilled borehole generally conforming to an arc. Again, there are typically borehole deviations from true arc configuration of a borehole segment drilled by sliding due to the same factors that cause borehole deviations from linear trajectory with drilling by rotating.
Many factors may combine to unpredictably influence the trajectory of a drilled borehole. It is important to accurately determine the borehole trajectory in order to determine the position of the borehole at any given point of interest and to guide the borehole to its geological objective. xe2x80x9cPosition,xe2x80x9d as that term is used herein in reference to boreholes, indicates the total vertical depth, longitude and latitude of a point of interest. Surveying of a borehole using existing methods involves the intermittent measurement of the earth""s magnetic and gravitational fields to determine the azimuth and inclination of the borehole at the BHA under static conditions; that is, while the BHA is stationary. These xe2x80x9cstaticxe2x80x9d surveys are generally performed at discrete survey xe2x80x9cstationsxe2x80x9d along the borehole when drilling operations are suspended to make up additional joints or stands of drillpipe into the drillstring. Consequently, the along hole depth or borehole distance between discrete survey stations is generally from 40 to 90 feet or more corresponding to the length of joints or stands of drillpipe added at the surface.
Reliable measurements of the earth""s magnetic and gravitational fields are available at the survey stations, and can be used to obtain reliable estimates of the azimuth and inclination of the borehole at the survey stations. Although the azimuth and inclination at a survey station of interest can be determined using measurements of the earth""s magnetic and gravitational fields, the depth cannot be measured, and must be determined by other means. The vertical depth and position of a survey station is determined by mathematically combining the segments of the borehole between discrete survey stations starting with the surface location of the drilling rig and progressing downward to the geologic objective of the borehole. The problem is that undetected borehole variations occurring between discrete survey stations cause substantial errors in calculating the vertical depth and position of a survey station of interest. Undetected borehole variations accumulate as mathematical combination of borehole segments is used to calculate and track borehole vertical depth and position.
It would be beneficial, therefore, to detect borehole variations and to accurately model the trajectory of the borehole between discrete survey stations in order to determine and track the spatial position of the borehole relative to the geologic objective of the borehole. Existing borehole survey techniques use various methods, including the tangential method, balanced tangential method, equal angle method, cylindrical radius of curvature method and the minimum radius of curvature method, to model the trajectory of the borehole segments between survey stations. Generally, these existing methods model the trajectory of the borehole segments between discrete survey stations based on the assumptions that either the azimuth and inclination remains constant from one survey station to the next or, more often, that the azimuth and inclination of the borehole smoothly transition from the values measured at one survey station to those measured at the next survey station. For example, the minimum radius of curvature method models the borehole segment between survey stations based on the assumption that the trajectory of the borehole segment conforms to an imaginary true arc whose length corresponds to the through-borehole distance from the nearest uphole survey station to the nearest downhole survey station. The length of the arc is assumed to be equal to the length of the drillpipe added to the drillstring during drilling of the borehole segment of interest. These imaginary arcs, one for each defined borehole segment, are stringed and mathematically combined together in sequence, from the known surface location of the drilling rig to the bottom of the borehole, in order to estimate the vertical depth and position of the borehole at any given point of interest.
The inability of these existing borehole survey methods to detect and account for borehole deviations that occur between survey stations is problematic. Existing methods of surveying boreholes, including the minimum radius of curvature method, introduce significant error that lead to inaccurate determinations of borehole vertical depth and position resulting in substantial losses of otherwise recoverable reserves due to inaccurate steering of directional boreholes. Borehole surveys using existing survey methods to model the trajectory of borehole segments between survey stations introduce substantial vertical depth and position error, and the extent of the error depends on the extent to which undetectable borehole variations occur between survey stations.
The vertical depth and position of a point in a borehole is determined using existing methods based on measurements of the earth""s magnetic and gravitation fields at survey location 32, the calculated depth, measurements of the earth""s magnetic and gravitational fields at survey location 31, and the length of drillpipe fed into the borehole between the survey station 31 and survey station 32. The depth error introduced by undetected borehole variations can be illustrated by reference to FIG. 1 which shows a borehole 14 and a borehole variation 33 occurring within the borehole segment between a first survey station 31 and a second survey station 32. The undetected borehole variation 33 will result in the actual depth of survey station 32 being substantially different from the depth calculated. The presence of the undetected borehole variation 33 renders the depth calculation for survey station 32 inaccurate, and the integration of the inaccurate depth for survey station 32 into subsequent determinations of the depth at later survey stations accumulates depth errors that ultimately reduces the recovery of targeted reserves.
The survey instruments that reliably measure the earth""s magnetic and gravitational fields at survey stations can also be used to obtain measurements of the earth""s magnetic and gravitational fields during drilling operations. Drilling operations, as that term is used herein, means that the drill bit is being rotated against rock. Literally thousands of measurements of the earth""s magnetic and gravitational fields can be obtained for each borehole segment using existing survey instruments. Successive measurements of the earth""s magnetic and gravitational fields during drilling operations may be separated by only fractions of a second or thousandths of a meter and, in light of the relatively slow rate of change of the magnetic and gravitational fields in drilling a borehole, these measurements are continuous for all practical analyses. For this reason, the determination of azimuth and inclination of a borehole from measurements of the earth""s magnetic and gravitational fields made during drilling operations are referred to herein as xe2x80x9ccontinuousxe2x80x9d measurements.
The problem is that the violent crushing and grinding of the drill bit against rock at the bottom of the borehole, the irregular interaction of the drillstring with the walls of the borehole, and even the constantly changing stresses in the connections between joints of drillpipe, all present during drilling operations, combine to contribute noise, shock and vibrations that severely contaminates continuously obtained measurements of the earth""s magnetic and gravitational fields to the extent that this data is not useful in reliably determining the azimuth and inclination of the borehole at points between survey stations. If continuously obtained magnetic and gravitational field data could be effectively used, borehole deviations occurring between survey stations could be detected and accounted for in calculating and tracking the depth of the borehole.
What is needed is a method of processing magnetic and gravitational field data obtained while drilling to make it useful in determining borehole azimuth, inclination and depth. What is needed is a method of continuously surveying the borehole during drilling in order to better detect and account for borehole variations resulting from gravity, geological heterogeneities, stiffness of the drillstring, interaction of the drillstring with the walls of the borehole and from transition effects of switching from sliding mode of drilling to rotation mode of drilling. What is needed is a method of either eliminating or dramatically reducing the time between reliable borehole surveys to prevent depth errors in directional drilling. What is needed is a method of continuously predicting future borehole trajectory using continuous borehole survey data, and of reconciling borehole trajectory predictions with borehole azimuth and inclination data obtained at survey stations in order to improve the accuracy of directional drilling. What is needed is a method of processing magnetic and gravitational field measurements obtained during drilling to reduce or eliminate the effects of shock, vibration and noise.
It is desirable that the method of continuously surveying the borehole while drilling use existing survey instruments and existing mud telemetry equipment commonly used with existing directional drilling methods. It is further desirable that the method of continuously surveying the borehole during drilling use existing data processing and computer equipment and software commonly used in connection with conventional borehole survey methods.
The present invention provides a borehole survey method that utilizes continuous measurements of the earth""s magnetic and gravitational fields obtained while actively drilling the borehole. The continuously obtained survey data is transmitted to the surface using mud telemetry systems. The continuously obtained survey data may be processed using downhole micro processors, or it may be first transmitted to the surface and there processed using computers to refine the data and eliminate or reduce error from unwanted shock, vibrations and nose from drilling. The present invention improves the accuracy of borehole spatial and positional computations by improving the accuracy of estimations of borehole vertical depth and position using integration of borehole surveys.
The objective of the present invention is to augment existing methods of modeling a drilled borehole, such as the minimum radius of curvature calculation, with effective estimates of the earth""s magnetic and gravitational fields obtained by processing measurements obtained while drilling. In one method of the present invention, a log of continuous azimuth and continuous inclination data is created, and the log is divided into sections delineated in time by discrete survey stations. The measurements of the earth""s magnetic and gravitational data within these delineated sections are then smoothed using windowed median smoothing to eliminate random noise caused by shock and vibration. Mathematical interpolation between adjacent smoothed magnetic and gravitational field measurements, or between adjacent smoothed azimuth and inclination data derived from magnetic and gravitational field measurements, is performed to determine a synthetic azimuth and inclination corresponding to a desired depth of interest corresponding to a survey station. The synthetic magnetic and gravitational field measurements, or the azimuth and inclination derived therefrom, is compared to the more reliable measurements of magnetic and gravitational fields obtained at survey stations in order to determine an offset correction, which is then used to adjust the continuously obtained magnetic and gravitational field measurements. Existing methods, such as the minimum radius of curvature calculation, may then be used to process the corrected measurements, and the resulting spatial and positional computations provide superior direction of the borehole to the desired target.